The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No. 11-355880, filed Dec. 15, 1999
Japanese Patent Application No. 2000-187228, filed Jun. 22, 2000
Japanese Patent Application No. 2000-370847, filed Dec. 6, 2000
1. Field of the Invention
This invention relates to a front tele-converter, and more particularly to a front tele-converter mounted on the object side of a photo-taking lens in order to enlarge its focal length, and also relates to its vibration-reduction technique.
2. Related Background Art
Front tele-converters conventionally used for video cameras are disclosed in, e.g., Japanese Patent Application Laid-open No.63-210810 and No.3-59508.
Vibration-reduction techniques for front tele-converters are also disclosed in Japanese Patent Publication No.7-119902 and Japanese Patent Application Laid-open No.9-171205.
The front tele-converter disclosed in Japanese Patent Application Laid-open No.63-210810, however, has had a disadvantage that it can attain a good imaging quality with difficulty because of its relatively small number of lenses and a simple construction.
The front tele-converter disclosed in Japanese Patent Application Laid-open No.3-59508 also has had a disadvantage that it is of low value in practical use because of its afocal magnification which is as low as about 1.46.
The front tele-converters disclosed in Japanese Patent Publication No.7-119902 and Japanese Patent Application Laid-open No.9-171205 still also have a relatively small number of lenses and a simple construction, also having a low afocal magnification, and can not be said to be high in the basic picture quality itself. In particular, the front tele-converter disclosed in Japanese Patent Publication No.7-119902 has an afocal magnification of as low as about 1.2 at best, and has had a disadvantage that it is of low value in practical use. Also, the front tele-converter disclosed in Japanese Patent Application Laid-open No.9-171205 has a simple construction that its vibration-reduction lens group consists of one negative lens, and hence has had a disadvantage that it can attain a good imaging quality with difficulty.
As a first basic embodiment of the present invention (hereinafter xe2x80x9cfirst inventionxe2x80x9d), an object of the present invention is to provide a front tele-converter that may cause less aberrations and has a superior imaging quality though it has a high magnification.
To achieve the above object, the first invention provides a front tele-converter having an afocal magnification higher than 1.9, which is detachably mountable to a photo-taking lens on its object side, wherein;
the front tele-converter has, in order from the object side, a positive lens group GF having a positive refractive power and a negative lens group GR having a negative refractive power;
the positive lens group GF has a positive cemented lens;
the negative lens group GR has a positive lens with its convex surface facing the image side; and
where a focal length of the negative lens group GR is represented by fF, and an effective aperture of a lens surface of the positive lens group GF on its side closest to the object side by "PHgr"F, the lens groups fulfill the condition of:
0.5 less than "PHgr"F/|fR| less than 10.0 
According to a preferred embodiment of the first invention, the positive lens group GF has a positive cemented lens consisting of combination of a negative meniscus lens disposed closest to the object side with its convex surface facing the object side and a positive lens;
the negative lens group GR has a positive lens disposed closest to the object side and a biconcave lens disposed on the image side of the positive lens; and
where a focal length of the positive lens group GF is represented by fF, and an effective aperture of a lens surface of the negative lens group GR on its side closest to the object side by "PHgr"R, the lens groups fulfill the condition of:
0.03 less than "PHgr"R/fF less than 1.0 
According to another preferred embodiment of the first invention, the positive lens group GF is constituted of, in order from the object side, a positive cemented lens consisting of combination of a negative meniscus lens with its convex surface facing the object side and a positive lens, and a positive meniscus lens; and at least one lens of the positive lens and the positive meniscus lens is formed of an optical glass having an Abbe number "ugr"d of 65 or more; and
where a focal length of the positive lens group GF is represented by fF, an afocal magnification of the front tele-converter by M, and an axial gap between the positive lens group GF and the negative lens group GR by DFR, the lens groups fulfill the condition of:
3.0 less than fF.M/DFR less than 15.0 
As a second basic embodiment of the present invention (hereinafter xe2x80x9csecond inventionxe2x80x9d), another object of the present invention is to provide a front tele-converter that can well correct various aberrations also at the time of vibration reduction and has a superior imaging quality and a superior vibration-reduction quality, though it has a high magnification.
To achieve the above object, the second invention provides a front tele-converter having vibration-reduction function, having an afocal magnification higher than 1.9, which is detachably mountable to a photo-taking lens on its object side, wherein;
the front tele-converter has, in order from the object side, a positive lens group GF having a positive refractive power and a negative lens group GR having a negative refractive power;
the positive lens group GF has a positive cemented lens;
the negative lens group GR has a cemented lens comprising a positive lens with its convex surface facing the image side;
a vibration-reduction lens group GV comprising a lens group consisting of part or the whole of the negative lens group GR and having a negative refractive power is movable in the direction falling at substantially right angles with the optical axis to effect vibration reduction; and
where the maximum amount of movement of the vibration-reduction lens group GV at the time of vibration reduction, in the direction falling at substantially right angles with the optical axis is represented by xcex94S, a focal length of the vibration-reduction lens group GV by fV, and a focal length of the positive lens group GF by fF, the lens groups fulfill the conditions of:
xcex94S/|fV| less than 0.2; and 
0.1 less than |fV|/fF less than 5.0 
According to a preferred embodiment of the second invention, where the maximum amount of movement of the vibration-reduction lens group GV in the direction falling at substantially right angles with the optical axis is represented by xcex94S, an effective aperture of a lens surface of the positive lens group GF on its side closest to the object side by "PHgr"F, and an effective aperture of a lens surface of the negative lens group GR on its side closest to the object side by "PHgr"R, the lens groups fulfill the condition of:
xcex94S/"PHgr"F less than 0.2; and 
2.0 less than "PHgr"F/"PHgr"R less than 10.0 
According to another preferred embodiment of the second invention, the positive lens group GF has a positive cemented lens consisting of combination of a negative meniscus lens disposed closest to the object side and a positive lens;
the negative lens group GR has a positive lens disposed closest to the object side and a biconcave lens disposed on the image side of the positive lens; and
where a focal length of the positive lens group GF is represented by fF, and an effective aperture of a lens surface of the negative lens group GR on its side closest to the object side by "PHgr"R, the lens groups fulfill the condition of:
0.03 less than "PHgr"R/fF less than 1.0 
According to still another preferred embodiment of the second invention, the vibration-reduction lens group GV comprises, in order from the object side, a cemented lens consisting of combination of a positive lens with its convex surface facing the image side and a biconcave lens; and
where an effective aperture of a lens surface of the vibration-reduction lens group GV on its side closest to the object side is represented by "PHgr"V, and an air gap between the positive lens group GF and the negative lens group GR along the optical axis by DFR, the lens groups fulfill the condition of:
0.000001 less than (xcex94S)2/DFR."PHgr"V less than 0.01 